This invention relates to an improved method of producing a soft fluororesin which is a graft copolymer comprised of an elastomeric fluorine-containing copolymer and a grafted crystalline fluoro-polymer.
Various fluororesins using crystalline polymers or copolymers are widely used by reason of their characteristic properties attributed to C-F bond, such as good heat resistance, high resistance to oils and many other chemicals and excellent weatherability. However, crystalline fluororesins are generally poor in flexibility or pliability, and therefore applications of such fluororesins are limited or rather unsuitable where flexibility is a matter of importance as in the cases of hoses, gaskets, seals and general-purpose sheets.
Where good flexibility is required besides the favorable properties of fluororesins it is usual to use fluororubbers. However, use of fluororubbers entails complicated operations because molding of a fluororubber needs to be preceded by kneading of a mixture of a raw fluororubber and additives such as stabilizers, fillers and cross-linking agents and needs to be followed by a cross-linking treatment at a controlled temperature. For the same reason there are limitations on the shapes of articles or parts to be formed of fluororubbers. In some cases it offers another problem that cross-linked fluororubbers can hardly be reworked.
U.S. Pat. No. 4,472,557 discloses fluorine-containing graft copolymers which can be regarded as intermediate between crystalline fluororesins and fluororubbers and have both the merits of crystalline fluororesins and the merits of fluororubbers. The graft copolymers are melt-processable and are soft and flexible. In a graft copolymer of this category the "trunk" polymer segment is a fluorine-containing elastomeric copolymer, and the "branch" segments are of a fluorine-containing crystalline polymer which may be a copolymer. In first preparing the elastomeric copolymer a peroxide of an unsaturated organic compound is used as a comonomer to introduce peroxy groups into the copolymer, and graft polymerization of the crystalline polymer segments is accomplished by utilizing thermal decomposition of the peroxy groups in the "trunk" polymer. In the obtained graft copolymer the grafted crystalline polymer segments act as a sort of physical cross-linking points at temperatures below the melting point of the crystalline polymer, so that the graft copolymer exhibits physical properties resembling the properties of a chemically cross-linked elastomer.
In a fluorine-containing graft copolymer of the above described type using a given combustion of an elastomeric copolymer and a crystalline polymer, the structure of the grafted crystalline polymer greatly affects the mechanical and thermal properties of the graft copolymer and also the degree of fluidity of the melted graft copolymer. In general, for obtaining a graft copolymer of this type excellent in both mechanical properties and thermal properties it is necessary that the grafted crystalline polymer is sufficiently high in the degree of polymerization and is high in crystallinity.
However, it is accepted that in graft polymerization of a crystalline polymer the growth of the polymer crystals is often obstructed by some factors so that the polymer does not fully exhibit its inherent crystallinity. This tendency becomes pronounced when the graft polymerization is carried out at a relatively high temperature. In producing the above described fluorine-containing graft copolymers it is usual to use t-butyl peroxyallylcarbonate to introduce peroxy groups into the "trunk" polymer. When this peroxide is used the subsequent graft polymerization reaction has to be carried out at about 100.degree. C., which is a considerably high temperature unfavorable for crystallinity of the grafted polymer.
It is conceivable to use a different peroxide which permits to carry out the graft polymerization reaction at a lower temperature to thereby increase the degree of polymerization of the crystalline polymer and reduce the amount of imperfect structure which is obstructive to good crystallinity of the polymer. Actually, however, it is difficult to use a peroxide which undergoes thermal decomposition at a sufficiently low temperature for the graft polymerization reaction. Since the peroxide is used as a comonomer in preparing the elastomeric copolymer as the "trunk" polymer, it is necessary to use a polymerization initiator which is active at a temperature lower than the thermal decomposition temperature of the peroxide in order to introduce undecomposed peroxy groups into the copolymer. Although such a low temperature active initiator may be embodied in a fluorinated compound such as heptafluorobutylic peroxide, such a compound is very inconvenient as an industrial material for several reasons including very limited supply of the starting fluorocarboxylic acid. It is also difficult to find an unsaturated peroxide which is available as an industrial material, is stably copolymerizable and, at the subsequent graft polymerization reaction, is active at a fairly low temperature. In the current practice, t-butyl peroxyallylcarbonate is almost the sole peroxide useful in producing the above described fluorine-containing graft copolymers. In conclusion it is unrealistic to select an unsaturated peroxide which decomposes at a very much lower temperature than t-butylperoxy allylcarbonate does.
U.S. Pat. No. 4,472,557 shows using 1,1,2-trifluoro-1,2,2-trichloroethylene as the liquid medium for the graft polymerization reaction. In our view, the use of such a solvent is also responsible for the difficulty of grafting a crystalline polymer which is sufficiently high in the degree of polymerization and in crystallinity. That is, the graft polymerization reaction, which is carried out at a relatively high temperature as mentioned above, is liable to be disturbed by chain transfer from the growing polymer into the solvent. The resultant insufficiency of the degree of polymerization is particularly significant when the crystalline polymer is polychlorotrifluoroethylene or a chlorotrifluoroethylene base copolymer, and in such a case the melting point of the obtained graft copolymer is far lower than an expected level.